Fuel cap

ABSTRACT

A fuel cap comprising a seal ring  1  mounted on a cap trunk portion  223  to realize liquid tightness and gas tightness, wherein the seal ring  1  includes a ring body  11  and an elastic support portion; the ring body  11  has a sizing relation between an external diameter Dc of the cap trunk portion  223  and an internal diameter Din of the ring body  11  which is Dc&lt;Din, and has a C-shaped section formed by a wedged circumferential groove opening outward in radial direction of the ring; and the elastic support portion is a plurality of the elastic members  12,13,14,15,16  or one of the elastic flanges  17,18 , or combination thereof, and disposed on the inner circumference side of the ring body  11  and protrudes with an amount of a radially inward protrusion d set at (Din−Dc)/2 or greater.

TECHNICAL FIELD

The present invention relates to a fuel cap for closing the filler neckof a feed oil pipe extending from the fuel tank of an automobile or thelike.

BACKGROUND OF THE INVENTION

The general fuel cap is constituted to have a seal ring mounted on a captrunk portion to be inserted into a filler neck, for example, asdisclosed in JP-A-2005-009661. This fuel cap realizes, when fastened onthe filler neck, a liquid tightness and a gas tightness by pressing theseal ring onto a cap sealing face, such as the lower face of the upperflange and the filler neck sealing face of the cap trunk portion, suchas the upper edge or the inner face of the filler neck. These liquidtightness and gas tightness are effectively exhibited when the centralaxis of the cap trunk portion is correctly aligned with the center ofthe seal ring. For this exhibition, the external diameter Dc of the captrunk portion and the internal diameter Din of the ring body are held ina sizing relation of Dc≧Din, and the seal ring is tightly mounted on thecap trunk portion. Thus, the seal ring is prevented from beingpositionally offset in the radial direction of the cap trunk portion.

SUMMARY OF THE INVENTION

The seal ring is made of synthetic rubber such as nitrile rubber orfluorine-containing rubber so that it absorbs the liquid fuel or itsvapor and to thereby swell. In other words, the internal diameter Din ofthe ring body becomes larger after the swelling than before theswelling. Therefore, the seal ring is designed such that the externaldiameter Dc of the cap trunk portion and the internal diameter Din ofthe ring body may take the sizing relation of Dc≧Din after the swelling.This implies that the internal diameter Din of the ring body is farsmaller than the external diameter Dc of the cap trunk portion when theseal ring is mounted. In order to prevent the positional displacement ofthe seal ring in the direction of the central axis of the cap trunkportion, however, the cap trunk portion is provided with the lowerflange paired to the upper flange as being the cap sealing face.Therefore, to mount the seal ring over the lower flange takes needlesstime and labor.

Even in the sizing relation of Dc≧Din appropriate for preventing thepositional displacement of the swelled seal ring in its radialdirection, moreover, this positional displacement is prevented byfastening the seal ring with respect to the cap trunk portion. Thisfastening of the seal ring on the cap trunk portion is one ofdisincentives for keeping the liquid tightness and the gas tightness, incase the internal pressure P in the feed oil pipe changes or in case theposition of the fuel cap comes out of balance.

Diameter of the seal ring is going to expand in a radially outwarddirection by increasing the internal pressure P in the filler neck whilebeing associated with the generation of the fuel vapor. In this case, itis desirable to enhance the liquid tightness and the gas tightness bythe seal ring so that especially the sealing face of the filler neck isdesigned to contact with the seal ring in a radial direction in whichthe seal ring is radially expanded. Thus, the seal ring having adiameter expanded radially outward is pressed more forcibly onto thesealing face of the filler neck thereby to exhibit the liquid tightnessand the gas tightness in proportion to the internal pressure P. At thisstage, however, the radial expansion of the seal ring led by theinternal pressure P is obstructed because of aforementioned fastening ofthe seal ring. Accordingly, there is still a problem that it is gettingharder to enhance the liquid tightness and the gas tightness in responseto an increase of the internal pressure P.

When the fuel cap is subjected to pried loads by an external forceapplied, on the other hand, a part of the cap is obliquely raised withrespect to the filler neck thereby to pull the cap sealing face awayfrom the filler neck sealing face. As a result, the raised side(hereinafter be referred to as a pried side) of the cap is pulled awayfrom the cap sealing face or the filler neck sealing face in response toinclination of the cap trunk portion, and thereby that loosening of theliquid tightness and the gas tightness may arise. Thus, the fuel cap inthe prior arts, in which the positional displacement of the seal ring isprevented to occur by fastening the seal ring on the cap trunk portion,has a problem that the liquid tightness and the gas tightness cannot bekept in necessary and sufficient condition when the pried loads applied.

The seal ring disclosed in JP-A-2005-009661 is that, in the procedure tofasten the fuel cap, the sealing portions formed on the upper face andthe inner circumference side and on the lower face of the seal ring aresequentially pressed onto the filler neck sealing face or the capsealing face thereby to realize the liquid tightness and the gastightness. However, even this seal ring of JP-A-2005-009661 has failedto solve the aforementioned problem of the mounting work or the problemthat the liquid tightness and the gas tightness become lower as the fuelcap is pried. In order to solve the aforementioned individual problems,therefore, it has been contemplated by investigating the seal ring todevelop the fuel cap which can exhibit the liquid tightness and the gastightness necessary and sufficient even when the pried loads applied.

As a result of the investigations, there is developed a fuel capcomprising a seal ring mounted on a cap trunk portion to realize liquidtightness and gas tightness, wherein the seal ring includes a ring bodyand an elastic support portion; the ring body has a sizing relationbetween an external diameter Dc of the cap trunk portion and an internaldiameter Din of the ring body which is Dc<Din, and has a C-shapedsection formed by a wedged circumferential groove opening outward inradial direction of the ring; and the elastic support portion isdisposed on the inner circumference side of the ring body and protrudeswith an amount of a radially inward protrusion d set at (Din−Dc)/2 orgreater. In this regard, the sizing relation between the externaldiameter Dc of the cap trunk portion and the internal diameter Din ofthe ring body, and the amount of the radially inward protrusion d of theelastic support portion belong to the design values in the seal ringpost-swelled.

In the fuel cap of the invention, the alignment between the central axisof the cap trunk portion and the center of the seal ring, that is, thepositioning role of the seal ring with respect to the cap trunk portionis taken by the elastic support portion to be brought into abutmentagainst the outer face of the cap trunk portion. Thus, the ring body ofthe seal ring is loosely fitted on the cap trunk portion so that thering body can be radially expanded by the internal pressure P to beapplied to a clearance ΔD formed by the elastic support portion (i.e.the clearance ΔD between the outer face of the cap trunk portion and theinner circumference side of the ring body). In this structure, moreover,when the pried loads applied, the internal pressure is applied from theclearance ΔD to the inner circumference side of the ring body thereby toelastically deform the elastic support portion positioned on the fulcrumside while being pried so that the ring body can move in the radialdirection of the cap trunk portion. Thus, the seal ring is released fromthe influences of the inclination of the cap trunk portion caused bybeing pried. In addition, the elastic support portion acts to suppressthe torsion of the ring body at the time when the seal ring is mountedon the cap trunk portion.

The specific elastic support portion can be exemplified by a pluralityof circumferentially arranged elastic ridges. In this case, the ringbody is supported through the elastic ridges abutting at their leadingend portions against the cap trunk portion, so that the seal ring ismounted on the cap trunk F portion. Thus, the ring body is so looselyfitted on the cap trunk portion that the individual elastic ridgesarranged interruptedly in the circumferential direction are brought attheir individual leading end portions against the outer face of the captrunk portion. In this regard, the ring body is so mounted on the captrunk portion as is deformed into a generally polygonal shape in a topplan view to have the elastic ridges at their corners, if the clearanceΔD set by the elastic ridges is set greater than (Din−Dc)/2. When thepried loads applied, moreover, the elastic flange positioned on thefulcrum side can be elastically deformed to move the ring body in theradial direction of the cap trunk portion.

The size, shape and number of the elastic ridges are basicallyarbitrary. Especially the shape may be such a cantilever shape as iseasy to be elastically deformed, because the shape affects to thedifficulty or ease of the elastic deformation. The specific elasticridges can be exemplified by vertical members extending in thecircumferentially perpendicular direction, or horizontal membersextending in the circumferential direction. The vertical members or thehorizontal members may be molded integrally with the ring body, or theseparate vertical members or horizontal members may be fixed to the ringbody.

In order to stabilize the support of the ring body, the elasticity ofthe elastic ridges may be suppressed. For example, the elastic ridgesmay be exemplified by cross members extending in themutually-perpendicular directions. These cross members are shaped bycombining the vertical members and the horizontal members. In order tofacilitate the elastic deformation, on the contrary, it is arbitrary touse cantilever support members or center-impeller support membersbridged in the circumferential direction as the elastic ridges. Thesecross members, cantilever support members or center-impeller supportmembers may be molded integrally with the ring body or separate crossmembers, cantilever support members or center-impeller support membersmay be so molded as are fixed on the ring body. These elastic ridges mayalso be a combination of elastic ridges of different shapes. In order tolocate the seal ring stably and precisely with respect to the cap trunkportion, it is desirable to dispose the elastic ridges in kindequiangularly on the inner circumference side of the ring body.

As a mode in which the horizontal members being the elastic ridges arecontinuously integrated, furthermore, the elastic support portion canalso be exemplified by an elastic flange extending on the inner side inthe circumferential direction of the ring body. This elastic flangesupports the ring body by having its inner circumference edge inabutment against the cap trunk portion thereby to mount the seal ring onthe cap trunk portion. Moreover, the seal ring can be moved in theradial direction of the cap trunk portion by elastically deforming theportion of the elastic flange on the fulcrum side when the pried loadsapplied. The elastic flange may be molded integrally with the ring body,or the individual elastic flange may be fixed on the ring body.

In order to prevent the torsion of the elastic flange itself, moreover,an auxiliary ring having a larger thickness than a thickness t of theelastic flange may be disposed at the inner circumference edge of theelastic flange. The seal ring using the elastic flange having theauxiliary ring as the elastic support portion is constituted such thatthe ring body and the auxiliary ring are aligned with each other, andsuch that the ring body and the auxiliary ring are jointed by theelastic flange. In this case, the elastic flange itself can be made tohave a smaller thickness t than that of the case, in which the auxiliaryring is not provided. The elastic flange and the auxiliary ring may alsobe molded integrally with the ring body. Moreover, an individualauxiliary ring may be fixed to the elastic flange, and this elasticflange may be fixed to the ring body.

The fuel cap of the invention mounts the seal ring which can becorrectly positioned while being loosely fitted on the cap trunk portionand which can be radially expanded by the internal pressure P in thefeed oil pipe. As a result, the fuel cap has effects to facilitate themounting work of the seal ring and not to deteriorate the liquidtightness and the gas tightness when the pried loads applied. Moreover,the present invention can be applied to the existing fuel caps only byexchanging the seal ring since the structure of the fuel cap in theinvention is similar but the particular of the seal ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a seal ring having vertical membersdisposed as elastic ridges on the inner circumference side of a ringbody;

FIG. 2 is a sectional view of the seal ring, taken along line a-a ofFIG. 1;

FIG. 3 is an expanded sectional view of a portion encircled in FIG. 2indicated by an arrow A;

FIG. 4 is a sectional view showing the state, in which a fuel cap havingthe seal ring of FIG. 1 mounted therein is fastened in a filler neck;

FIG. 5 is a sectional view showing the state, in which a fuel cap havingthe seal ring of FIG. 1 mounted therein is pried in a filler neck;

FIG. 6 is a fragmentary top plan view of the seal ring having horizontalmembers disposed as the elastic ridges on the inner circumference sideof the ring body;

FIG. 7 is an expanded sectional view corresponding to FIG. 3 showing theseal ring of FIG. 6;

FIG. 8 is a fragmentary top plan view of the seal ring having crossmembers disposed as the elastic ridges on the inner circumference sideof the ring body;

FIG. 9 is an expanded sectional view corresponding to FIG. 3 showing theseal ring of FIG. 8;

FIG. 10 is a fragmentary top plan view of the seal ring havingcantilever support members disposed as the elastic ridges on the innercircumference side of the ring body;

FIG. 11 is an expanded sectional view corresponding to FIG. 3 showingthe seal ring of FIG. 10;

FIG. 12 is a fragmentary top plan view of the seal ring havingcenter-impeller support members disposed as the elastic ridges on theinner circumference side of the ring body;

FIG. 13 is an expanded sectional view corresponding to FIG. 3 showingthe seal ring of FIG. 12;

FIG. 14 is a fragmentary top plan view of the seal ring having anelastic flange disposed as an elastic support portion on the innercircumference side of the ring body;

FIG. 15 is an expanded sectional view corresponding to FIG. 3 showingthe seal ring of FIG. 14;

FIG. 16 is a fragmentary top plan view of the seal ring having anauxiliary ring disposed as the elastic support portion on the innercircumference side of the ring body; and

FIG. 17 is an expanded sectional view corresponding to FIG. 3 showingthe seal ring of FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described in the following withreference to the accompanying drawings.

A fuel cap 2 of an embodiment has a general structure including an outercap 21 and an inner cap 22, as shown in FIG. 4. A seal ring 1 is mountedon a cap trunk portion 223 locating between an upper flange 221 formounting the outer cap 21 thereon and a lower flange 222 pairing up withthe upper flange 221. The fuel cap 2 clamps and crushes the seal ring 1between the lower face of the upper flange 221, a cap sealing face 23and a filler neck sealing face 31 so that the liquid tightness and thegas tightness are worked effectively.

As shown in FIGS. 1 to 3, the seal ring 1 is molded of synthetic rubberintegrally with: a ring body 11 of a C-shaped section having an upperand a lower lip portions 112,113 across a wedged circumference groove111 on the outer circumference side thereof; and a plurality of verticalmembers 12 as the elastic ridge disposed on the inner circumference sideof the ring body 11. The ring body 11, as shown in FIG. 4, exhibits theliquid tightness and the gas tightness by pressing the upper lip portion112 onto the cap sealing face 23 and pressing the lower lip portion 113onto the filler neck sealing face 31 while the fuel cap 2 is fastened ina filler neck 3. The vertical member 12 includes a base portion 121 asbeing of the outer edge of the member having an arc shape correspondingto that of the inner circumference face of the ring body 11, and aleading end portion 122 as being of the inner edge of the member havinga straight shape corresponding to that of the outer face of the captrunk portion 223.

The ring body 11 has a sizing relation between the external diameter Dcof the cap trunk portion 223 and the internal diameter Din of the ringbody 11 which is Dc<Din, in the design values after having swelled.Moreover, the vertical members 12 protrude from the ring body 11 with anamount of a radially inward protrusion d set at (Din−Dc)/2, in thedesign values after having swelled. The proper protrusion d of thevertical members 12 is about 0.5 to 1.5 mm, preferably about 1.0 mm. Asa result, the seal ring 1 is mounted in the fuel cap 2, as apparent fromFIG. 1, such that the leading end portions 122 of the vertical members12 abut against the outer face of the cap trunk portion 223 thereby tosupport the ring body 11 through the vertical members 12. In otherwords, the ring body 11 is loosely fitted on the cap trunk portion 223thereby to form a clearance ΔD between the outer face of the cap trunkportion 223 and the inner circumference side of the ring body 11.

In this embodiment, eight vertical members 12 are disposed at an equalinterval in the circumferential direction. The individual verticalmembers 12 have the same shape and size so that the seal ring 1 can haveits center aligned with the central axis of the cap trunk portion 223when the leading end portions 122 of all the vertical members 12 arebrought into abutment against the outer face of the cap trunk portion223. Meanwhile, no restriction is made on the number of the verticalmembers 12. In order to position the seal ring 1 precisely and stablywith respect to the cap trunk portion 223, however, it is preferablethat the number of the vertical members 12 goes to three through twelve.This description on the setting of the number and interval of thevertical members 12 can likewise apply to the other elastic ridges, aswill be described hereinafter. Since the ring body 11 is loosely fittedon the cap trunk portion 223, moreover, the seal ring 1 can be mountedon the cap trunk portion 223 more easily than the seal ring of the priorarts, in which the ring body 11 is fastened on the cap trunk portion223.

The clearance ΔD to be formed between the individual vertical members 12acts, when the fuel cap 2 is secured, to prevent the ring body 11 frombeing tightened to the cap trunk portion 223 caused by the swelling.Simultaneously, the clearance ΔD also acts to apply a rising internalpressure P in the feed oil pipe to the inner circumference side of thering body 11 thereby to expand the ring body 11 radially outwarddepending on the internal pressure P. This will enable to press theupper and lower lip portions 112,113 more forcibly to the cap sealingface 23 and the filler neck sealing face 31 respectively so that theliquid tightness and the gas tightness can be enhanced depending on theinternal pressure P.

The function of the clearance ΔD, in which the internal pressure Papplied through the clearance ΔD to the inner circumference side of thering expands diameter of the ring body 11, leads to an advantage thatthe liquid tightness and the gas tightness can be retained necessarilyand sufficiently even while the fuel cap 2 is being pried. When the fuelcap 2 is pried, as shown in FIG. 5, the cap sealing face 23 and thefiller neck sealing face 31 leave each other on the pried side. In theseal ring 1 of the invention, however, it is possible to keep the state,in which the internal pressure P is applied from the clearance ΔD to theinner circumference side of the ring to radially enlarge the pried sideportion of the ring body 11 thereby to press the upper and lower lipportions 112,113 further onto the cap sealing face 23 and the fillerneck sealing face 31, respectively. Thus, the liquid tightness and thegas tightness of the fuel cap 2 are ensured necessarily andsufficiently.

On the fulcrum side when the pried loads applied, on the other hand, thecap sealing face 23 and the filler neck sealing face 31 come close toeach other thereby further to crush the ring body 11, so that the upperand lower lip portions 112,113 are pressed onto the cap sealing face 23and the filler neck sealing face 31, respectively, and thereby toenhance the liquid tightness and the gas tightness. Even if, however,the internal pressure P is applied through the clearance ΔD to the innercircumference side, the ring body 11 on the fulcrum side is hardlyradially enlarged. When the internal pressure P increases, the verticalmembers 12 positioned on the fulcrum side are rather elasticallydeformed to move the seal ring 1 toward the pried side in the radialdirection. Thus, the upper and lower lip portions 112,113 on the priedside are pressed more forcibly by the cap sealing face 23 and the fillerneck sealing face 31 thereby to ensure the liquid tightness and the gastightness depending on the internal pressure P. As a result, thethickness t of the vertical members 12 may be set within a range of 0.5mm to 1.5 mm in the case of using the seal ring 1 made of nitrile rubberhaving a spring hardness of 50, for example, so that the verticalmembers 12 can be elastically deformed.

The seal ring 1 to be mounted on the fuel cap 2 in the present inventionis required functions of which the seal ring 1 can be positioned withrespect to the cap trunk portion 223 by providing the elastic supportportion on the inner circumference side of the ring body 11, and ofwhich the ring body 11 can be radially enlarged easily by applying theinternal pressure P to the clearance ΔD formed by the elastic supportportion. In this regard, elastic ridges or an elastic flange havingshapes in place of that of the exemplified vertical members 12 can alsobe used as the elastic support portion, as will be described in thefollowing.

The seal ring 1 shown in FIGS. 6 to 7 has a plurality of horizontalmembers 13, of which the aforementioned vertical members 12 are turnedby 90 degrees, mounted as the elastic ridges at an equal interval on theinner circumference side of the ring body 11. The horizontal members 13are exemplified to have a shape of an isosceles triangle in a top planview, and the apex of this isosceles triangle as a leading end portion131 is to abut against the outer face of the cap trunk portion 223. Asshown in FIGS. 6 and 7, moreover, the horizontal members 13 can also beformed into a trapezoidal shape in a top plan view having a wide leadingend portion 131 formed into an arc-shape along the outer face of the captrunk portion 223. Both types of the horizontal members 13 are requiredto have a high shaping precision at the leading end portion forpositioning the seal ring 1 with respect to the cap trunk portion 223.What to adopt any of the horizontal members 13 having those individualshapes may be selectively chosen in accordance with the conditionsdemanded for the individual products, such as the molding cost, theelastic performance required, and so on. An amount of the protrusion dof the horizontal members 13 may be set at (Din−Dc)/2 or greater, as inthe case of the aforementioned vertical members 12. Moreover, thethickness t of the horizontal members 13 may also be set within therange of 1.0 mm to 1.5 mm, in case the seal ring 1 is made of nitrilerubber of a spring hardness of 50, for example.

These vertical members and horizontal members can be used incombination. In this modification, the vertical members and thehorizontal members may be disposed alternately. In addition, a pluralityof cross members 14 formed by combining the vertical member and thehorizontal member, as shown in FIGS. 8 and 9, may be adopted as theelastic ridges. In case that these cross members 14 are used, an innerside edge of a vertical portion 141 including an apex formed by crossingthe vertical portion 141 and a horizontal portion 142 as a leading endportion 143 abuts to the outer face of the cap trunk portion 223. Thesecross members 14 are constituted to suppress the elastic deformations asthe elastic ridges. An amount of the protrusion d of the cross member 14may be set at (Din−Dc)/2 or greater, as well as the aforementionedelastic ridges. Moreover, the thickness t of the vertical portions 141and the horizontal portions 142 may be set to an equal thickness t or adifferent thickness t within the range of 0.5 mm to 1.5 mm, in the caseof the seal ring made of nitrile rubber of a spring hardness of 50, forexample.

The seal ring 1 shown in FIGS. 10 and 11 employs a plurality ofcantilever support members 15 that can be relatively easier toelastically deform, as the elastic ridges. The cantilever support member15 is constituted by cutting off the inner circumference side of thering body 11 into a square shape in a top plan view, to form a concavepart 114, and by fixing a base portion 152 formed at one end of a bentelastic member portion 151 to one end side of the concave part 114. Thecantilever support member 15 supports the ring body 11 elasticallyagainst the cap trunk portion 223 by bringing the bent portion of theelastic member portion 151 as a leading end portion 153 into abutmentagainst the outer face of the cap trunk portion 223. An amount of theprotrusion d of the cantilever support member 15 is set at (Din−Dc)/2 orgreater, and the length L and the thickness t of the cantilever supportmember 15 are adjusted depending on the elastic force required. Thethickness t, for example, may be set within the range of 2.0 mm to 4.0mm in the case of the seal ring 1 made of nitrile rubber of a springhardness of 50.

The elastic ridge having stronger elastic force is exemplified by theelastic ridge formed by a center-impeller support member 16, as shown inthe seal ring 1 of FIGS. 12 and 13. This center-impeller support member16 is constituted by fitting a bent elastic member 161 in the concavepart 114 which is formed by cutting off a portion of the ring body 11 onthe inner circumference side into a square shape in a top plan view. Thecenter-impeller support member 16 is constituted by fixing base portions162,162 formed at the two ends of the elastic member portion 161, to therespective two ends of the concave part 114 and by bridging the elasticmember portion 161 over the concave part 114. The center-impellersupport member 16 supports the ring body 11 elastically to the cap trunkportion 223 by bringing the bent portion of the elastic member portion161 as a leading end portion 163 into abutment against the outer face ofthe cap trunk portion 223. In this embodiment, a reinforcing rib 164 isformed on the inner side of the leading end portion 163 so as toreinforce the strength at the leading end portion 163. An amount of theprotrusion d of the center-impeller support member 16 may be set at(Din−Dc)/2 or greater. Moreover, the length L and the thickness t of thecenter-impeller support member 16 are adjusted depending on the elasticforce required. The thickness t, for example, may be set within therange of 1.0 mm to 4.0 mm in the case of the seal ring 1 made of nitrilerubber of a spring hardness of 50.

In place of the elastic ridges continued intermittently incircumferential direction, moreover, an elastic flange 17 integratedcontinuously in the circumferential direction can be used as the elasticsupport portion, as shown in FIGS. 14 and 15. This elastic flange 17 hasnot only elasticity but also resiliency. Unlike the elastic ridges,moreover, the elastic flange 17 has an inner circumference edge 171continuing all over the circumference so that the seal ring 1 isprecisely positioned especially with respect to the cap trunk portion223. However, this elastic flange 17 has to be elastically deformed whenthe pried loads applied. For this necessity, the elastic flange 17, forexample, may be set within the range from the combination of theprotrusion d=0.8 mm and the thickness t=0.5 mm to the combination of theprotrusion d=1.5 mm and the thickness t=1.0 mm in the case of the sealring 1 made of nitrile rubber of a spring hardness of 50.

In order to prevent the torsion due to the flexibility of the elasticflange, an auxiliary ring 181 having a thickness larger than thethickness t of an elastic flange 18 may be disposed on the innercircumference edge 182 of the elastic flange 18, as shown in the sealring 1 of FIGS. 16 and 17. Then, the auxiliary ring 181 suppresses theflexibility of the elastic flange 18 thereby to prevent the torsion ofthe elastic flange 18. As a result, it is possible to reduce thethickness t of the elastic flange 18. Specifically, the elastic flange18 containing the auxiliary ring 181, for example, may be set within therange from the combination of the protrusion d=0.8 mm and the thicknesst=0.3 mm to the combination of the protrusion d=1.5 mm and the thicknesst=0.8 mm in the case of the seal ring 1 made of nitrile rubber of aspring hardness of 50.

1. A fuel cap comprising a seal ring mounted on a cap trunk portion torealize liquid tightness and gas tightness, wherein: the seal ringincludes a ring body and an elastic support portion; the ring body has asizing relation between an external diameter Dc of the cap trunk portionand an internal diameter Din of the ring body which is Dc<Din, and has aC-shaped section formed by a wedged circumferential groove openingoutward in radial direction of the ring; and the elastic support portionis disposed on the inner circumference side of the ring body andprotrudes with an amount of a radially inward protrusion d set at(Din−Dc)/2 or greater.
 2. A fuel cap according to claim 1, wherein theelastic support portion includes a plurality of elastic ridgescircumferentially arranged, and wherein the elastic ridges are verticalmembers extending in circumferentially perpendicular direction.
 3. Afuel cap according to claim 1, wherein the elastic support portionincludes a plurality of elastic ridges circumferentially arranged, andwherein the elastic ridges are horizontal members extending incircumferential direction.
 4. A fuel cap according to claim 1, whereinthe elastic support portion includes a plurality of elastic ridgescircumferentially arranged, and wherein the elastic ridges are crossmembers extending in mutually-perpendicular direction.
 5. A fuel capaccording to claim 1, wherein the elastic support portion includes aplurality of elastic ridges circumferentially arranged, and wherein theelastic ridges are cantilever support members bridged in circumferentialdirection.
 6. A fuel cap according to claim 1, wherein the elasticsupport portion includes a plurality of elastic ridges circumferentiallyarranged, and wherein the elastic ridges are center-impeller supportmembers bridged in circumferential direction.
 7. A fuel cap according toclaim 1, wherein the elastic support portion is an elastic flangeextending in circumferential direction of the ring body.
 8. A fuel capaccording to claim 1, wherein the elastic support portion includes anelastic flange extending in circumferential direction of the ring body,and an auxiliary ring disposed at the inner circumference edge of theelastic flange.